Product Description
The WS159 water soluble solder paste is designed to provide humidity tolerance and excellent wetting to meet the requirements for reliable solder joints in Printed Circuit Board (PCB) Assemblies.
WS159 offers high levels of repeatability and consistency even in a wide range of temperatures (65-85F) and relative humidity (25-65% RH). WS159 is formulated to deliver exceptional cosmetics with easy-cleaning residues using warm water.

Printing
Stencil aperture design and stencil quality are major factors in achieving excellent print consistency with any water washable solder paste. Laser Cut stencils from Stencils Unlimited are recommended for optimal print performance, and can be custom designed to minimize and improve the yields of any process. Some general stencil aperture design guidelines follow:

Discrete components
A 0.002” reduction (L & W) for water washable and a 0.002” reduction (L & W), with “U-shaped” home-plates, for no clean to minimize mid-chip solder beads.

Printer Operation

Solder Paste Bead Size:

2cm (~0.75”) on startup

Add when bead < 1.4 cm (~0.5”)

Squeegee:

Metal, Slic™ blade preferred

60 degrees from horizontal

Speed

25 to 150 mm/sec. (1 to 6 in/sec)

Adjust printer for a pull or print demand process

Pressure:

0.18-0.27 Kg/cm (squeegee length)

Underside Wipe:

Slic™ and Ultraslic™ stencils should exceed >10 prints/wipe

Stencil life / Environment:

>6 hours

30+65% RH and 20-25°C

Storage and Handling

Cartridges should be stored tip down.

Paste can be stored up to two weeks at room temperature.

To prolong the shelf life of WS159, refrigerate between 5°C~10°C, (41°F~50°F). At this range the shelf life will exceed 6 months.

WS159 should not be allowed to freeze.

When refrigerated, water soluble solder paste must be allowed to warm up to room temperature.

Paste must be >= 22°C, (~66°F) prior to applying to stencil for processing.

Working range of WS159 is between 22-32°C, (~66°F - 89.5°F).

First-In-First-Out (FIFO) inventory management practices should be used with all water soluble pastes.

Heating
A linear ramp of 0.7°C to 2.0°C C/second is suggested to gradually remove the solvents and other volatile components in the water washable paste.
This also helps in minimizing solder balls, beading and bridging from hot slump.
A linear ramp also helps minimize depletion of flux activity which can occur with excessive temperature, preheat times, and at very high reflow temperatures.
A profile with a soak between 200-210°C for less than 20 seconds can be used to reduce void formation on BGA and CSP devices. (Request FCTA’s profile guide to void reduction).
While a linear profile typically does not produce tombstoning, a short 10-20 second dwell prior to transitioning into the liquidus point of the solder, and minimizing the QR between soak and liquid temperatures will help minimize tombstoning if experienced.

Cooling
A cooling rate of 2°C-3°C per second is typical for most Sn62 and Sn63 applications. These parameters should be utilized to insure a fine grain solder structure and minimal IMC layer.